Viewers complained about the use of strong language in the episode, particularly because it aired on Christmas Day.
The complaints referred to a sketch featuring an elderly woman, Nan Taylor, who regularly says the catchphrase, "what a fucking liberty".
Other viewers complained that a separate sketch, featuring a family from Northern Ireland giving each other balaclavas and knuckle-dusters for Christmas, portrayed the family in a stereotypical way.
In response to the complaints, the BBC argued that the general tone of the programme is very well recognised and that several Catherine Tate characters are now iconic figures in their own right, including Nan Taylor the swearing pensioner.
The BBC said it was aware that the composition of audiences on Christmas Day is not typical, with a higher proportion of children, but the broadcaster said it aired the show at 10.30pm that day, well after the watershed.
It also said that it played a pre-transmission announcement that alerted viewers to the content of the programme, which said: "Plenty of presents still under the tree here on BBC1, including this. If the little ones are still up, be warned, there's strong language in the 'Catherine Tate Christmas Show'".
Where the family from Northern Ireland was concerned, the BBC argued that, given the degree of exaggeration in this series of sketches, they could not reasonably have been interpreted as an attempt to stereotype any community in Northern Ireland. It believed the sketches employed an established comedy tool of exaggerating an extreme attitude or situation.
Ofcom agreed that the 'Catherine Tate' characters are well-established and that the Christmas special was typical of the normal series and was in keeping with viewers' expectations.
The watchdog ruled that because the BBC broadcast the programme on Christmas day well after the watershed, as well as giving a pre-transmission warning, any possible offence was justified by the context.
It also dismissed the argument that the Northern Irish family was stereotyped as a typical Irish family, as the sketches were "absurd in the extreme", which would have been clear to the audience.
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